Light-emitting diodes (LEDs) are often used for various types of lighting devices, illumination, electronic equipment, and the like, for their high energy conversion efficiencies and long lives. An LED capable of emitting visible light is generally fabricated using AlGaInN (hereinafter typified by GaN) or a Group III-V compound semiconductor material of AlGaInP GaN is used for LEDs which emit green light, blue light, or ultraviolet light. AlGaInP is used for LEDs which emit red light, orange light, or yellow light.
At present, GaN crystal is grown on a sapphire (Al2O3) substrate in consideration of, for example, cost and quality. In the GaN layer grown on the sapphire substrate, however, threading dislocation serving as a high-density nonradiative recombination center occurs in GaN crystal because of lattice mismatch between a sapphire crystal lattice and a GaN crystal lattice. Accordingly, an optical output (i.e., an external quantum efficiency) and a durability life decrease, and a leakage current increases.
In addition, GaN has an refractive index of about 2.4 and the sapphire substrate has an refractive index of about 1.8 in the wavelength range of blue light, i.e., the difference in refractive index between GaN and the sapphire substrate is large in this wavelength range. Accordingly, about 70% of light emitted from an InGaN/GaN multi-quantum well layer is confined in the GaN layer including the multi-quantum well layer due to total internal reflection, and is either self-absorbed in the multi-quantum well layer during transmission in the GaN layer, or absorbed in, for example, an electrode, to be finally converted into heat. That is, total internal reflection due to the difference in refractive index greatly reduces the light-extraction efficiency of an LED.
To reduce threading dislocation as described above, or to increase the light-extraction efficiency, a technique for growing a GaN layer and an AlGaN layer using a so-called patterned sapphire substrate (PSS) provided with unevenness formed by previously etching a surface of a sapphire substrate on which a GaN layer is to be grown is proposed (see, for example, PATENT DOCUMENT 1).